Displays are ubiquitous and are a core component of wearable devices, smart phones, tablets, laptops, desktops, TVs and display systems. Common display technologies today range from Liquid Crystal Displays (LCDs) to more recent Organic Light Emitting Diode (OLEO) Displays and Active Matrix Organic Light Emitting Diode Displays (AMOLEDs).
The display architectures include passive and active matrix displays depending on whether each pixel is driven separately or not. Active drive circuitry uses thin film transistor (TFT) technology where transistors based on amorphous silicon (A-Si), low temperature polysilicon (LTPS) and amorphous Indium Gallium Zinc Oxide (IGZO) technology are manufactured on glass panels which may have glass substrate sizes from first generation displays of around 30 cm×40 cm to the latest tenth generation displays (known as GEN10) of around 2.88 m×3.15 m.
In most portable devices (i.e. battery powered devices) the display uses a majority of the available battery power. Additionally, the most common user complaint for portable devices is insufficient display brightness. To extend battery life and improve brightness levels it is necessary to develop new display technologies that reduce power consumption and produce higher luminance emission from the light source.
Inorganic LEDs (ILEDs) are emerging as the third generation of flat display image generators based on superior battery performance and enhanced brightness. The ILED Display is at a basic level a variation of the OLED (organic LED) display. The OLED concept is based on passing current through organic or polymer materials that is sandwiched between two glass planes to produce light. The proposed ILED Display concept essentially replaces the organic LED material with a discrete standard LED (which is made of inorganic materials) at each pixel of the display (each pixel consists of three individual Red, Green and Blue LEDs for colour displays).
Standard (i.e. inorganic) LED devices have been around for many years and their performance (efficiency, brightness, reliability and lifetime) has been optimised over many years as the LED industry has pursued many commercial opportunities—especially the challenge of developing LED technology to enable it to replace the standard incandescent bulbs for general light applications, i.e. inorganic LEDs are significantly more efficient, bright and reliable than the new and less developed OLED materials.
The concept of individually switchable standard LEDs (R, G & B) at each pixel in a display is well known. This approach is in widespread use for large information displays. However, to-date it has not been possible to scale this approach down to smaller displays as standard LEDs are typically planar chips which are inefficient for light direction control. Additionally, the assembly of the many millions of pixels needed for a laptop or smart phone display is not feasible at this scale using traditional assembly manufacturing techniques.
The current challenges with ILED display manufacture are significant and assembly techniques to overcome wafer yields losses need to be factored in to the manufacturing strategy of ILED displays for today's yields and higher anticipated yields in the future. Selective pick up tools (PUTs) is one solution to overcoming yield problems where defective die are identified and replaced at source. Depending on the yield, it may not be practical or economical either to replace known bad die or to transfer only KGD from a wafer to a temporary carrier for pick to the TFT substrate. Both approaches require wafer level testing to determine KGD or defective chips on the wafer, which is complicated.
Smart assembly processes with resolutions to manipulate and handle small die improve ILED assembly on the glass panel. There is therefore a need for an assembly process with high throughput that can enable massive parallel pick and transfer of ILED dies of size <10 μm a side from the native LED wafer onto a glass TFT substrate at accuracies approx ±3 μm or less.
Smart assembly methods are being developed in the industry for ILED displays and range from “non selective” elastomer conformal stamps, laser assisted transfer, direct self-assembly methods, fluidic assembly and selective MEMs based printheads. All techniques require the preparation of assembly ready chips where the bulk of the substrate is removed or the epilayer released from the substrate. For ILED displays to become a commercial reality, many or all of the above challenges need to be solved.